The present invention relates to a seal for rotating shaft adapted for use in, for example, a compressor for a car air conditioner.
In general, a sealing fluid used in a compressor for a car air conditioner include a refrigerant (gas or oil) and refrigerator oil, which are kept at a predetermined pressure. The sealing fluid must be prevented from leaking without regard to the state, rotating or nonrotating, of the rotating shaft.
Conventionally, there is a rotating shaft seal which is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 4-157264. As shown in FIG. 1, this rotating shaft seal is mounted between a hole of a housing and a rotating shaft (not shown). It integrally comprises a first sealing element a formed of an elastic rubber material and a second sealing element b formed of a synthetic resin (e.g., fluoroplastic (PTFE). The first sealing element a includes a sealing lip portion d which is in contact with the outer peripheral surface of the rotating shaft. The second sealing element b includes a sealing lip portion e which is located at the back of lip portion d and in contact with the outer peripheral surface of the shaft. These two sealing elements are needed because the second sealing element b alone cannot prevent a gas leakage (especially when the rotating shaft is at a standstill), although it can stop a liquid leakage.
Moreover, a plurality of annular grooves f are formed on that surface of the sealing lip portion e of the second sealing element b which is in contact with the rotating shaft, the grooves f extending along the circumference of the shaft. A high contact surface pressure is obtained in the regions defined between the grooves f, so that the sealing performance of the sealing lip portion e is improved.
Conventionally, however, the sealing lip portion d of the first sealing element a is in the form of a sharp edge finished by cutting. If the rotating shaft is rotated under pressure with the sealing lip portion (trim lip) d pressed against the outer peripheral surface of the shaft, therefore, the lip portion d exerts a great force of pressure on the shaft, that is, it produces a high sealing effect. Thus, good lubrication cannot be secured between the sealing lip portion and the outer peripheral surface of the rotating shaft.
Accordingly, the first sealing element a is heated to high temperature, and cannot enjoy a long life. Also, the lubrication between the sealing lip portion e of the second sealing element b and the outer peripheral surface of the rotating shaft is poor. Thus, the sealing lip portion e is liable to be worn out or heated to high temperature.